In a nuclear reactor, such as boiling water reactor (BWR) for example, nuclear fuel bundles are arranged in a reactor core for heating water to generate steam. Each of the fuel bundles includes a plurality of longitudinally extending fuel rods arranged in a suitable array such as a square array of nine-by-nine fuel rods. Of course, the number of fuel rods in any given fuel bundle design may be conventionally selected, and some of the fuel rod positions may instead include conventional water rods of the same or larger diameter as the fuel rods as desired.
Each of the fuel rods includes a plurality of tubular nuclear pellets contained in an elongate tubular cladding which is typically made of Zircaloy. The outer surface of the cladding has a preferred smooth finish characterized by the absence of scratches since scratches in the cladding can accelerate corrosion of the cladding which is undesirable. The cladding may be scratched during assembly of the fuel bundle when the individual fuel rods are axially inserted through their respective spacer grids having cells which contain bosses and/or springs provided to securely clamp the individual fuel rods at various intermediate locations to prevent movement and vibration thereof during operation in the nuclear reactor.
More specifically, a typical fuel bundle includes an upper tieplate and a lower tieplate at opposite longitudinal ends of the fuel bundle, and a plurality of longitudinally spaced apart spacer grids disposed therebetween. Each of the spacer grids includes a plurality of cells configured for receiving and supporting respective ones of the fuel rods. The cells may take any conventional configuration including square, circular, or other shapes for surrounding the fuel rod. Each of the cells typically includes a plurality of bosses or stand-offs which center the fuel rod within the cell. The cells typically also include a suitable spring such as a leaf spring which clamps the fuel rod against the bosses to prevent movement such as vibratory movement during operation of the fuel bundle in the reactor core.
In order to assemble the individual fuel rods into the fuel bundles, each fuel rod is typically inserted horizontally through each of the spacer grids in turn, and, therefore, slides along the bosses and the springs of the respective cells. Such sliding can scratch the fuel rod cladding which can lead to undesirable corrosion during operation.
In order to prevent such scratching of the fuel rods during the assembly process, various conventional techniques are utilized. For example, the fuel rod may have a suitable coating over its cladding to act as a lubricant and to prevent scratching of the cladding itself during the assembly process. It is also known to use metal or polyester protective sleeves in the respective cells so that sliding of the fuel rods through the cells does not cause scratching thereof. Conventional protective sleeves are typically utilized individually and typically extend longitudinally through the spacer grids which typically number about seven or eight, for example, or the sleeves may be arranged in groups extending from a common handle. Furthermore, suitable additional devices are sometimes required to compress the cell springs for allowing the fuel rods to be inserted through the cells.
These various conventional configurations have disadvantages including the need to assemble fuel rods near the center of the spacer grids prior to assembly of the outer rows of fuel rods to allow access to the protective sleeves for the removal thereof after assembly of the fuel rods. Since the protective sleeves must necessarily be relatively thin and flexible for allowing the fuel rods to be inserted in the cells without binding or damage thereto, and for allowing the sleeves to be unfurled from the fuel rods after assembly, the sleeves are, therefore, easily worn and damaged as well as being potentially distorted during assembly which increases the difficulty of inserting the fuel rods during the assembly procedure.